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  <meta charset="UTF-8">
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  <title>Document</title>
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<body>
  <script>
    let arr = [1, 2, 3, 4, 5, 6]
    const res = arr.indexOf(5)
    console.log(res);
    function selfindexOf(arr, cb) {
      let state=-1
      // 先遍历数组每一项
      for (let i = 0; i < arr.length; i++) {
        if(cb===arr[i]){
          return state=i
        }
      }
      return state
    }
    const arr2 = selfindexOf(arr, 4)
    console.log(arr2);
  </script>
</body>

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